1. Field of the Invention
The present invention relates to a sensor with a bridge structure to detect an incident light.
2. Description of the Related Art
In a sensor device, e.g. an infrared sensor, a lot of sensor elements are arranged in a two-dimensional matrix, and used as an image sensor for obtaining an image of infrared rays. The infrared sensor is recently manufactured on a semiconductor substrate to have a very small size using a micromachining process, which is also used in an IC manufacturing process.
FIG. 1 is a perspective view illustrating the structure of a conventional infrared sensor of a non-cooling type which is manufactured using the micromachining process. FIG. 2 is a plan view of the conventional infrared sensor.
In the conventional infrared sensor, a light detecting section 2 is formed on a semiconductor substrate 1 to be spaced from the substrate 1. A sensor resistor pattern 5 is formed of a material whose resistance value changes depending upon temperature, on the surface of the light detecting section 2. Outputting sections 11 and 12 are connected to the light detecting section 2 and leg sections 3 and 4 are connected to the outputting sections 11 and 12, respectively. The detecting result of the light detecting section 2 is connected to a circuit (not shown) which is formed on the substrate 1, through the outputting sections 11 and 12 and the leg sections 3 and 4.
As seen from FIGS. 1 and 2, the non-cooling type infrared sensor can operate at room temperature. The non-cooling type infrared sensor can be made small in size, compared with a cooling type infrared sensor, because the cooling mechanism is unnecessary. However, the sensor sensitivity of the non-cooling type infrared sensor is low. Therefore, improvement of the sensor sensitivity becomes an important technical problem.
In a bolometer type infrared sensor, a change of the resistance value caused by the heat of the infrared rays inputted to the light detecting section 2 is taken out as an electric signal, and at least one of the following matters must be implemented to improve the sensor sensitivity. That is, (1) a quantity of infrared rays which are absorbed by the light detecting section 2 is increased, (2) noise which is generated in the light detecting section 2 is reduced, and (3) the heat insulation of the light detecting section 2 is improved. In order to improve a responsivity in addition to the sensor sensitivity, it is necessary to decrease thermal capacity of the light detecting section 2.
A high temperature coefficient (TCR) of the resistance and small noise are required as important characteristics of the material of the sensor resistor pattern 5. The temperature coefficient (TCR) and resistivity of the sensor resistor pattern 5 are in a trade off relation. Resistance noise and 1/f noise of noises to which the material of the sensor resistor pattern 5 is related becomes a problem in a practical use. Such resistance noise is dependent on the resistance value of the sensor resistor pattern 5. When the light detecting section 2 is made large in size, the resistance value increases so that it is difficult to decrease the resistance noise. Therefore, the resistivity of the material of the sensor resistor pattern 5 is required to be as small as possible. In this case, the temperature coefficient (TCR) of the resistor becomes small because of the above-mentioned trade off relation. As a result, it is difficult to improve the sensor sensitivity.
In order to decrease the thermal capacity of the light detecting section 2, it could be considered that a material with a small specific heat is used for the light detecting section 2, that the film thickness of the light detecting section 2 is made thin, and that the area of the light detecting section 2 is made small. However, because silicon oxide or silicon nitride is generally used for the light detecting section 2 for the reason of the manufacturing process, materials which can be used for the light detecting section 2 are limited. When the light detecting section is spaced from the substrate 1, a problem would be caused from the viewpoint of structural strength, if the film thickness of the light detecting section 2 is made thin. Therefore, in order to decrease the thermal capacity of the light detecting section 2, it is desirable to decrease the area of the light detecting section 2.
In order to increase the absorption quantity of the infrared rays by the light detecting section 2, it could be considered to increase the area of the light detecting section 2. However, when the area of the light detecting section 2 is increased, the infrared sensor itself must be made large in size. Also, the resolution is degraded. Further, the increase in size is contradictary to decreasing of the thermal capacity of the light detecting section 2.
Also, when the light detecting section 2 is spaced from the substrate 1, it is difficult to support the light detecting section 2 with a large area. Therefore, use of a lens has been proposed to increase the absorption quantity of the infrared rays by the light detecting section 2 without changing the area of the light detecting section 2. Such an infrared sensor is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-113352).
According to this reference, the infrared rays are collected by the lens, and are irradiated to the light detecting section. However, if the infrared rays are merely collected by the lens and irradiated to the light detecting section, the collected infrared rays pass through the light detecting section without sufficient absorption of them by the light detecting section, when the light detecting section is thin. Because the thickness of the light detecting section is as thin as about 0.1 to 1.0 .mu.m, the infrared rays with a wavelength of about 8 to 12 .mu.m can not be sufficiently absorbed by a single layer film with the discussed thickness. Therefore, in order to improve the absorption quantity of the infrared rays, some measure is necessary.
Also, in order to increase the sensor sensitivity of the light detecting section 2, it is necessary to improve heat insulation of the light detecting section 2 from the substrate 1. For this purpose, it could be considered that the outputting sections 11 and 12 and the leg sections 3 and 4 are formed of material with a low thermal conductivity. Also, it could be considered that the outputting sections 11 and 12 and the leg sections 3 and 4 are made thin, narrow and long, so that these section have high thermal resistance. At this time, if the outputting sections 11 and 12 and the leg sections 3 and 4 are made thin, narrow and long too much, problems, occurs in relation to the strength of the outputting sections 11 and 12 and the leg sections 3 and 4.